The present invention relates generally to the field of microanalysis, particularly to the field of Fourier-transform-infrared microspectrometry, and specifically to reflection techniques of infrared microspectrometry. Spectroscopic analysis using radiant energy in the infrared region of the spectrum is a primary technique for chemical analysis. Since the introduction of commercial infrared microspectrometers, the advantages of being able to both view the sample area with a visible-light microscope and define microscopic areas for analysis by limiting the viewing field for spectral measurement has proven to be of great value. Infrared microspectrometer systems, disclosed in U.S. Pat. No. 4,877,747 to Donald W. Sting and Robert G. Messerschmidt, have been used for an ever-expanding range of applications. They have been used to detect and identify trace contaminants, to analyze multilayered composite structures such as packaging materials, to analyze micro-electronic devices, to establish phase distributions in polymeric materials, to identify inclusions in minerals, and so forth.
These microscope systems analyze samples in the nanogram to picogram weight range. The sampling area ranges from 100.times.100 micrometers (.mu.m) to 10.times.10 .mu.m. The goal is to analyze the smallest possible sampling area. This invention is based on the recognition that most microanalysis is performed on samples that are visible to the unaided eye. For these samples, the present microscope systems are too costly and too complex for the analyst. The infinity-corrected microspectrometer accessory of this invention uses infinity-corrected optics to produce a system without remote field apertures, and incorporates an integral video-imaging system for video microscopy. This system fills an important need in microanalysis, reduces the system's cost, and is easier for the analyst to use.
The microscope described in the Sting and Messerschmidt patent has been the standard configuration for infrared microspectroscopy. This established the need for both transmission and reflection capabilities and for remote-image-plane masks. At this time, all commercial microscopes use remote-image-plane masks to define sample areas.
A primary requirement of a true infrared microscope system for microspectrometry is that it should be able to perform transmission measurements and/or reflection measurements. Transmission infrared spectroscopy has been the primary mode for collecting spectral data. However, reflection techniques have advantages over transmission techniques because of the minimum requirements for sample preparation for reflection measurements. Reflection techniques make the analyses of samples much less expensive.
Reflection-only microspectrometer systems also greatly reduce the cost and complexity of a micro-sampling accessory. In microspectrometer systems that allow both transmission and reflection measurements, two optical systems have been required. One system delivers radiant energy to the sample and collects reflected radiant energy, while a second system is required to detect radiant energy transmitted through the sample. The dual functions of transmission and reflection greatly complicate the optical design.
Internal-reflection microspectroscopy provides certain advantages over both transmissive techniques and external reflection microspectroscopy, particularly in the ability to analyze thick samples. With the introduction of internal-reflection microspectrometry, as shown in U.S. Pat. No. 5,093,580 to Donald W. Sting, and U.S. Pat. No. 5,200,609 to Donald W. Sting and John A. Reffner (also known as attenuated total reflection microspectrometry or micro-ATR) reflection microspectrometry has gained even greater importance.
Prior to this invention, all micro-sampling accessories with viewing capabilities have used conventional microscope optics (in the form of an objective, an eyepiece, and a direct-viewing head) with an option of attaching either a photographic camera or a video camera. However, these accessories required additional optical elements so that the eyepiece and viewing head could be positioned at a convenient location. Provisions for attaching a photographic or video camera also added to the cost of the accessory. By eliminating a number of optical elements and other parts needed in earlier accessories to achieve the same capability, this invention simplifies and reduces the cost of the accessory.
Most samples analyzed by infrared spectroscopy are a microgram (.mu.g) or larger in size. A .mu.g of material is generally just barely visible, whereas most samples analyzed via infrared spectroscopy are detectable with the unaided eye. With this invention, these .mu.g-sized samples may be analyzed easily via internal or external reflection, with little or no sample preparation. By microsamples, we mean samples or areas of large specimens that generally represent 0.05-50 .mu.g of solid or liquid material. In this infinity-corrected microsampling accessory, the area of the sample analyzed is determined by the detector element's size, the detector optics, the source size, the source optics, and the objective optics. Commercially available Fourier-transform spectrometer benches have radiant-energy sources that are several millimeters in their smallest dimension. Since the source optics of commercial FT-IR spectrometers usually have focal lengths greater than the focal length of the objective lens, the image of the source on the sample is generally much larger than the projected size of the detector on the sample. In this invention, it is usually the detector size and its associated optics that determine the size of the sample being analyzed by the radiant energy. This is a unique feature of the infinity-corrected optical system of this invention. In all commercially-available infrared microspectrometer systems, field apertures are used to define the size of the sampling area. In this invention, the detector and source (together with associated optics) define the sampling area. This infinity-corrected system is further simplified, in comparison with presently available microspectrometer accessory systems, because the collimated radiant energy from the source and the radiant energy reflected from the sample, which is recollimated by the objective lens, can be directed along the optical path by plane mirrors, positioned at any of a number of positions along the optical path. The use of plane mirrors, and the freedom to select positions for these mirrors based on structural and manufacturing considerations, instead of the optical considerations which dictated the design of current instruments, make the accessory of this invention less expensive and more robust.
It is an object of the present invention to provide an infrared microspectrometer accessory that allows: (a) visual examination by an integrated video system, (b) external-reflection spectral analysis, and (c) internal-reflection spectral analysis.
Another object is to use infinity-corrected reflecting objectives both to provide a means for directing radiant energy onto a microscopic area and to allow visualization of the magnified image of the specimen through an integral video system.
A further object is to provide a simplified system for internal-reflection spectroscopy of microscopic samples with the same objective-lens used to view the microscopic sample. The internal-reflection element is inserted into the light path to allow internal-reflection spectral measurement.
Yet another object is to provide an internal-reflection element that reduces the need to control the angle of incident radiation by objective lens optical design.
A still further object of the present invention to provide a simplified system for detecting contact of a sample with an internal-reflectance element.
Other objects of this invention will be apparent from the following description, which is provided to enable any person skilled in the art to make and use the invention, and which sets forth the best mode contemplated by the inventors of carrying out their invention. Various modifications to the specific embodiment disclosed herein, within the general principles of the invention as defined herein, will be apparent to those skilled in the art.